The present invention relates to an analyzing apparatus for assaying trace impurity contents in a gas, more particularly to such an analyzing apparatus for determining efficiently trace impurity contents in various kinds of high-purity gases on the ppb to sub ppb levels.
There have conventionally been utilized a gas chromatograph with a photoionization detector, a gas chromatograph mass spectrometer, a Fourier transform infrared spectrophotometer with a long light path gas cell, etc. when various kinds of impurity contents present on the ppb levels in high-purity gases employed in semiconductor manufacturing processes are assayed. Meanwhile, as monofunctional spectrometers, there have been utilized a yellow phosphorus emission type trace oxygen spectrometer, an emission spectrometer for assaying nitrogen in argon, various kinds of trace moisture meters, etc.
Recently, a sensitive gas analyzer called atmospheric pressure ionization mass spectrometer (APIMS) is utilized. This spectrometer is an evaluation analyzer indispensable for detection of impurity contents in high-purity gases as a spectrometer capable of measuring impurity contents on the ppb (1/1,000,000,000) to ppt (1/1,000,000,000,000) levels, and such atmospheric pressure ionization mass spectrometers are now used to determine impurity contents in nitrogen, argon, hydrogen or helium on the ppb to sub ppb levels.
However, there are some kinds of high-purity gases and impurity contents which are theoretically difficult to determine using the atmospheric pressure ionization mass spectrometer. It is theoretically difficult to perform sensitive determination of impurity contents, for example, hydrogen and carbon monoxide in nitrogen and most of impurity contents in oxygen. When determination of hydrogen in nitrogen is carried out using an atmospheric pressure ionization mass spectrometer, for those sample gases which contain water and methane as coexistent impurity contents, the protons (H) of water and methane are also detected in the mass number 29 (N2H) detecting hydrogen, making it difficult to carry out accurate assay.
Meanwhile, carbon monoxide is determined by detecting the mass number 12 of C (carbon) atom, and coexistence of large amounts of methane (CH4) and carbon dioxide (CO2) each having a carbon atom makes it impossible to distinguish the carbon atom of carbon monoxide from those of such impurity contents. Accordingly, it has been a prerequisite of assay when carbon monoxide in nitrogen is actually determined that these coexistent impurity contents are as small as possible. However, since actual nitrogen gases contain such impurity contents as described above and in various levels, assay of impurity contents in nitrogen required an extra analyzer which can determine accurately hydrogen and carbon monoxide in addition to the atmospheric pressure ionization mass spectrometer.
Further, it is essential as a matter of fundamentals in the determination using an atmospheric pressure ionization mass spectrometer that the ionization potential of the major component gas is greater than those of impurity contents. However, the atmospheric pressure ionization mass spectrometer involves a theoretical problem that, when impurity contents in a high-purity oxygen gas are assayed, the ionization potential of the major component gas oxygen is small (12.6 eV), so that detectable impurity contents are limited to those which have small ionization potential values compared with that of oxygen, making it impossible to detect nitrogen, carbon monoxide, carbon dioxide, methane, etc. all having greater ionization potential values.
Accordingly, when such impurity contents are to be determined, analyzers each having a combination of means for separating impurity contents from oxygen gas using separation columns packed with various kinds of fillers and means for detecting impurity contents (photoionization detector, mass spectrometer, etc.), i.e., a gas chromatograph with a photoionization detector and a gas chromatograph mass spectrometer (GCMS), are employed under the present circumstances.
Further, as shown in FIG. 1, although there is devised a gas chromatograph atmospheric pressure ionization mass spectrometer using the atmospheric pressure ionization mass spectrometer (APIMS) 52 described above as detector for a gas chromatograph (GC) 51 aiming at sensitive assay, there are very few cases where it is employed practically. Meanwhile, referring to moisture content in oxygen, it is difficult to separate moisture on the ppb level using the gas chromatograph 51, so that a sensitive moisture meter 53 is generally connected, in addition to the gas chromatograph 51, to the atmospheric pressure ionization mass spectrometer via a selector valve 54 so as to determine moisture content separately. While there is also proposed a method for assaying moisture content in oxygen using the atmospheric pressure ionization mass spectrometer resorting to the cluster reaction, it had been difficult to apply this method to impurity contents other then moisture and hydrocarbons (ethane, propane, etc.).
As described above, while the atmospheric pressure ionization mass spectrometer is involved somehow or other in carrying out assays of impurity contents in high-purity gases, conditions under which a sample gas is introduced to the spectrometer varies between the case where determination is carried out using the atmospheric pressure ionization mass spectrometer only and the case where determination is carried out using additionally a gas chromatograph installed on the upstream side of the spectrometer. Accordingly, when the former determination is followed by the latter determination, the spectrometer is stopped, and after selection of the sample inlet for the gas chromatograph, the spectrometer is started up. Since determination is carried out after running-in of the spectrometer to provide a stable state, it takes considerable labor and time.
Further, as described above, considering that all of the impurity contents in various kinds of gases cannot be determined with high sensitivity (ppb to sub ppb) using the atmospheric pressure ionization mass spectrometer only, the above system involves inconveniences in that it must employ a plurality of analyzers, that adjustment of each analyzer is intricate and that the assay takes much time.
Therefore, it is an objective of the present invention to provide an analyzing apparatus for assaying trace impurity contents, integrated with an atmospheric pressure ionization mass spectrometer useful for determination of trace impurity contents on the ppb to sub ppb levels in various kinds of high-purity gases and a gas chromatograph, thus enabling assay of various kinds of impurity contents in various kinds of high-purity gases efficiently.
The analyzing apparatus for assaying trace impurity contents in a gas according to the present invention, which is provided with a gas chromatograph and an atmospheric pressure ionization mass spectrometer, comprises a system for introducing a sample gas introduced from a sample gas introduction source directly to the atmospheric pressure ionization mass spectrometer; a system for introducing the sample gas via the gas chromatograph to the atmospheric pressure ionization mass spectrometer; and channel selecting means for changing over the channel of the sample gas to either of these two systems.
Further, the channel selecting means is provided, as sample gas channels, with an assay passage for introducing the sample gas to the atmospheric pressure ionization mass spectrometer and a purge passage for exhausting the sample gas; a pressure control mechanism or a flow regulating mechanism, for equalizing the pressure of the sample gas when it flows through the assay passage and that of the sample gas when it flows through the purge passage, being installed in the purge passage or in the purge passage and an inlet channel or outlet channel of the atmospheric pressure ionization mass spectrometer.
According to the analyzing apparatus of the present invention, all the impurity contents in a sample gas can be determined by changing over the sample gas introduction passage to the desired one, and particularly when impurity contents in an oxygen gas are to be assayed, other analyzers need not be used. This not only eliminates the necessity of installing a plurality of analyzers and adjusting them but also saves the time and labor to be spent for the analysis. Further, those analyses which have conventionally be performed using the atmospheric pressure ionization mass spectrometer and other analyzers can be carried out using a unit of analyzing apparatus in a short time, and also high sensitivity and high accuracy determination on the ppb to sub ppb levels can be carried out. In addition , analyses can be performed in stable states by controlling pressure fluctuation and the like which can occur in channel selection.